Patient Preparation for Outpatient Blood Work and the Impact of Surreptitious Fasting on Diagnoses of Diabetes and Prediabetes

OBJECTIVE: To describe patient preparation for routine outpatient blood work and examine the implications of surreptitious fasting on interpretation of glucose results. PATIENTS AND METHODS: We designed a survey and administered it between September 1, 2016, and April 30, 2017, to assess fasting behaviors in a convenience sample of 526 adults presenting for outpatient blood work in 2 health systems between 7 am and 12 pm. We reviewed the electronic health records to extract glucose results. We describe the frequency of clinician-directed fasting and surreptitious fasting. In those surreptitiously fasting, we describe the frequency of missed diagnoses of prediabetes and diabetes. RESULTS: Of 526 participants, 330 (62.7%) self-identified as fasting, and 304 (92.1%) of those fasting met American Diabetes Association fasting criteria. Only 131 (24.9%) of those fasting were told to fast by their health care team. Almost 50% (257 of 526) believed it was important to fast for every blood test. Of the 64 patients with diabetes who were taking insulin, 37 (57.8%) fasted and took their insulin as prescribed. Among the 89 patients without diabetes who fasted without knowledge of their health care team and had glucose tested, 2 (2.2%) had a missed diagnosis of diabetes and 18 (20.2%) had a missed diagnosis of prediabetes. CONCLUSION: Fasting for outpatient blood work is common, and patients frequently fast without awareness of their health care team. Failure to capture fasting status at the time of glucose testing is a missed opportunity to identify undiagnosed cases of diabetes and prediabetes.